Recently, there has been a remarkable increase in the interest in environmental and energy technologies, and a technology for measuring a current value of electrical equipment with high precision in a non-contact manner is necessary in many situations. For example, a current sensor is essential for an inverter that is a representative device of power semiconductors and is essential for a breaker as well.
Since the above-described devices consume a current of hundreds of amperes, in a case where a magnetic field (current-induced magnetic field) that is induced by the current is to be detected in a non-contact manner, the magnitude of the magnetic field to be measured by the current sensor used for the detection is a one-digit number to tens of oersteds (Oe).
In addition, in a case where a current sensor is used in car electronics, since an environmental noise inside an engine room is high, it is difficult to use an MI sensor that is saturated although it has an extremely high sensitivity. Accordingly, in such a case, a non-contact current sensor using a giant magnetoresistance (GMR) element or a tunneling magnetoresistance (TMR) element is considered to be used.
The above-described GMR element and the TMR element have already been practically used as read heads of a hard disk. The GMR element and the TMR element have a magnetic multi-layered film structure called a spin valve. Here, the spin valve structure is a structure that is made by stacking a magnetic layer (pinned layer), a non-magnetic layer, and a magnetic layer (free layer). The pinned layer is configured such that the magnetization direction is difficult to be reversed more than that of the free layer. The non-magnetic layer is arranged so as to achieve the function of cutting out the magnetic coupling between two magnetic layers (between the pinned layer and the free layer). The spin valve structure operates as a GMR element in a case where a metal is used for the non-magnetic layer and operates as a TMR element in a case where an insulator is used.
As the operating principle of the GMR element and the TMR element, a phenomenon is used in which the magnetization direction of the free layer has an angle with respect to the magnetization direction of the pinned layer due to an effect of a magnetic field which is a detection target. In other words, since the resistance value of the spin valve structure changes in accordance with the angle, the GMR element and the TMR element detect a magnetic field in accordance with the change in the resistance value. Here, the resistance value is the lowest in a case where the magnetization direction of the free layer and the magnetization direction of the pinned layer are parallel to each other (parallel arrangement), and the resistance value is the highest in a case where the magnetization direction of the free layer and the magnetization direction of the pinned layer are anti-parallel to each other (anti-parallel arrangement).
In order to detect a minute leakage of the magnetic field from the detection target, the GMR element and the TMR element are based on an analog operation for detecting a magnetic field in an area, which has linearity, between the parallel arrangement and the antiparallel arrangement.
For example, a ferromagnetic film (called a hard bias film) is arranged to be adjacent to in a magnetic head. In other words, a predetermined bias magnetic field is applied due to this arrangement, and a state is formed in advance in which the magnetization direction of the free layer is inclined with respect to the magnetization direction of the pinned layer. By using the initially inclined state as a reference, the above-described analog operation is realized. In addition, since the sensitivity of a device is determined based on the coercive force of the free layer, in the GMR element and the TMR element, the materials or the structure is designed such that the coercive force of the free layer is lower than the magnetic field as the detection target.